1. Field of the Invention
The present invention relates to a magnetic balance type current sensor utilizing a magnetoresistance effect element (TMR element or GMR element).
2. Description of the Related Art
In an electric automobile, a motor is driven using electricity generated by an engine, and the intensity of a current used for driving the motor is detected by, for example, a current sensor. The current sensor includes a magnetic core placed around a conductor and having a cutaway portion (core gap) formed at a portion thereof, and a magnetic detecting element placed in the core gap. As such a current sensor, for example, there is a magnetic balance type current sensor as illustrated in FIG. 23. An example of the current sensor is described in Japanese Unexamined Patent Application Publication No. 8-15322.
The magnetic balance type current sensor illustrated in FIG. 23 includes a feedback coil 2 wound around a magnetic core 1 in a direction cancelling a magnetic field B1 generated by a current I to be measured. In the magnetic balance type current sensor, if the current I to be measured flows, an output voltage occurs in a magnetic detecting element 3 owing to the magnetic field B1 according to the current, and a voltage signal output from the magnetic detecting element 3 is converted into a current and fed back to the feedback coil 2. A magnetic field (cancelling magnetic field) B2 occurring owing to the feedback coil 2 and the magnetic field B1 occurring owing to the current I to be measured cancel out each other, and hence an operation is performed so that a magnetic field constantly becomes zero. At this time, the feedback current flowing through the feedback coil is subjected to voltage conversion and extracted as an output. In FIG. 23, a reference symbol 4 indicates an amplifier, and R indicates a detection resistor.
In recent years, electric automobiles have had high power and high performance, and hence current values to be dealt with have become large. Therefore, it is necessary to avoid magnetic saturation at the time of a large current. While it is necessary to enlarge a magnetic core in order to avoid the magnetic saturation, when the magnetic core is enlarged, there occurs a problem that the current sensor itself grows in size. In order to solve the problem of the current sensor utilizing such a magnetic material core, there has been proposed a magnetic balance type current sensor utilizing a magnetoresistance effect element without utilizing a magnetic material core. An example of the magnetic balance type current sensor is described in PCT Japanese Translation Patent Publication No. 2000-516714.